The BBC has an article celebrating 20 years of mobile phones. The event they are referring to is the signing of the GSM agreement, which went on to produce GSM (2nd generation mobile phones). The article also quotes the figures that it took 12 years for the number of GSM phones to reach 1 billion, and 30 months for it to reach 2 billion.
More typically, people tend to date the first mobile phone network to 1981, with the Nordic Mobile Telephone network. With the first GSM network being in Finland in 1991.
But the key takeaway, for me, from the article is to remind us of the accelerating change we are going through. When I started working in the Market Research industry in 1978, there were no mobile phones, no laptops, no faxes, and the only personal computers were Apple IIs, with a memory of 64K. Until about 10 years ago most of the world had never seen a telephone, within a few years we will find that most of the world's population own a mobile phone.